The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre/ Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. EP-A-423399 and EP-A-410500.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR-element) or in the image-receiving layer of a so-called single-support element, also called-mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
According to the preferred embodiment of the mono-sheet DTR-offset printing plate, disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-P-1,241,661, a flexible support e.g. paper or an organic resin is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the now-useless emulsion layers.
Offset printing plates with a flexible support are preferred in short-run jobs over plates with an aluminium support because plates with a flexible support are less expensive. Furthermore, when the flexible support is also transparent e.g. an organic resin such as polyethylene terephtalate an exposure through the back is possible.
When using said type of mono-sheet DTR offset printing plate it has been found that the visual inspection of the printing plate is difficult because the printing areas have a shining metallic outlook, whereas the non-printing areas are black. Furthermore these printing plates are not compatible with other types of printing plates in regard to dampening solutions and printing inks, what is cumbersome for the printer. As for other printing plates it is required that the mono-sheet DTR offset printing plates have good ink acceptance in the printing areas and no ink acceptance in the non-printing areas (no staining). It is furthermore desirable that the number of copies that have to be disposed off because of ink acceptance in the non-printing areas (so called toning) during start-up of the printing process is limited.